Neuro-radiology for medical graduates

Various imaging modalities used for investigating neurological disorders are:

X-Rays ( Plain radiography)
Ultrasonography
CT scan of the brain & Spine, CT scan with contrast, CT Angiography
MRI Brain and Spine, MR Angiography, MRI with contrast, MR Spectroscopy, MR Tractography, Functional MRI of the brain, MR cisternography
Angiography, Digital substraction angiography ( DSA)
Myelography, CT myelogram
PET CT Scan
PET MRI
TCD ( Transcranial Doppler)
SPECT

Plain Radiography

X-Ray skull is useful for the diagnosis of skull bone osteomyelitis, Craniovertebral junction abnormalitise, Tumors of the cranial bones like osteomas, osteosarcoma,  metastasis to skull may be seen on skull X - ray films. Skull fractures in head injury & Growing skull fractures in children are diagnosed on skull radiography.

CT scan: CT scan is commonly used abbreviation of Computed Axial Tomography ( CAT ) scanning. This investigation machine was developed in 1970s and it was a most important development in the field of Neuroradiology after the development of X rays ( 1890s) and angiography ( 1920s and 1930s). It is a non invasive procedure and uses X-rays for the imaging. It utilizes X-Ray beam which passes through the tissue and produces a picture like x ray but in varying shades of grey.  The density of tissue changes the picture. CT scan produces axial or cross sectional ( slices) images of the body.
Computer  measures the density of the tissue through which x ray beam passes. CT scan machine uses multiple pencil beams of x ray which rotate in the gantry and pass through the body and on opposite side dosimeter measures the amount of radiation reaching it. Each cubic part of tissue is known as voxel ( in New machines about 512 voxels) . Each voxel produces a pixel. Computer measures the attenuation of the beam and assigns a Hounsfield Unit  ( HU ).
Sir Godfrey Hounsfield from England and Allan McLeod Cormack from USA shared Nobel prize in 1979 for invention of CT scan . All shades of Gray for image May be assigned a number ' HU'. Any HU value below minus 15 will appear pure black on CT film and any HU value above 155 HU will appear pure white.  Common  HU values are water zero ( 0 )
CSF in brain 10 to 16, Air minus 1000, Fat minus 60 to minus 120. Fat containing medullary bone will appear less white as compared to compact cortical bone ( HU +1000).


CT scan of the brain is the investigation of choice
For brain trauma patients, because
- it is less time consuming,
-the presence of the metal ( bullet in gun shot injury, metal in stab injury) is not a contraindication,
- an trauma patient where the history of pace maker of heart or metallic implant is not known , CT scan is possible,
-detects bony injuries, like a fracture, depressed fracture and hematoma associated with fracture
-better delineation of hematoma .
CT scan brain is also an investigation of choice
For detecting subarachnoid hemorrhage ( spontaneous subarachnoid hemorrhage) due to rupture of an intracranial aneurysm

CT angiography ( CTA ) is an investigation to detect the aneurysm of the brain. It has become an important tool for detecting the site of aneurysm bleed, location , and other characteristics of the aneurysm of the brain . It is more sensitive than MR Angiography and its sensitivity is comparable to the Digital Substarction Angiography.

CT scan of the spine: Although MRI of the spine is undoubtedly the investigation of choice for spine, CT scan of the spine is still an important investigation. CT scan of the spine is required when MRI of the spine is not possible, for example, if a patient is with metal prosthesis ( spinal instrumentation with ferromagnetic material like steel), or a metallic bullet is impinged in the spinal cord following a gun shot injury. CT spine also helps in conditions like canal stenosis, bony fractures, ossified posterior longitudinal ligaments, etc.

High resolution CT scan, 3D reconstruction, CT myelogram , Perfusion Coputed Tomography , Intraoperative CT scan are other applications of CT scan.

MRI is the most important development in the field of neuroradiology after the development of X- rays, Angiography, and CT scan.
MRI is a non invasive radiological investigation. It does not expose the patient to the risk of radiation. It uses magnetic field . It provides multiplanar images, i.e, images in sagittal, coronal and axial planes.
Functional MRI is another non invasive investigation which helps in imaging of the eloquent area of the brain.
MR Spectroscopy provides the clue about the nature of the lesion and helps in identifying infective and neoplastic lesions of the brain.
Intraoperative MRI is an advanced technique for intraoperative imaging of the lesions inside the operation theater.
How to interpret MRI brain images?
MRI images are usually black & white. There are T1 weighted, T2 weighted, FLAIR , Diffusion weighted images and if contrast is given then T1 contrast images.
To identify T1 weighted image, see the ventricles. lateral Ventricles are in the center and contain CSF. 
On CT usually only Axial images are seen but on MRI Axial, Coronal and sagittal images are seen.
On T1 weighted image, CSF will appear Black (Hypointense).
On T2 weighted image , CSF will appear White ( Hyperintense).
On FLAIR ( Flow Attenuation Inversion Recovery) the intraventricular CSF will appear Black but brain edema will appear White.
Contrast images are usually T1 contrast Images. So, CSF will appear Black and some lesions like Meningioma will become white ( Hyperintense) after contrast enhancement.





PET and SPECT are nuclear neuroimaging and help in physiological assessment. PET ( Positron Emission Tomography) is further advanced to utilize CT or MRI imaging techniques and known as PET- CT or PET-MR. PET is used for detecting metastasis and recurrence of the tumor. PET scan commonly  utilizes Flurodeoxy glucose ( FDG) which is a radioactive tracer.

Digital substraction angiography ( DSA) is invasive investigation  which involves introducing a catheter and injecting intravenous contrast into the femoral artery. It is the gold standard investigation for defining an intracranial aneurysm, Arteriovenous malformation ( AVM) , vasospasm after Subarachnoid hemorrhage ( SAH) and other diseases of intracranial vasculature.

TCD ( Transcranial Doppler ): Noninvasive investigation to detect the vasospasm in a case of SAH.
          Although Ultrasound is not a good investigation to detect intracranial pathologies as ultrasound waves do not cross bones, there are certain places where bone is very thin like temporal squama or areas in cranium which have windows like orbit. So, the flow of blood through the intracranial arteries may be detected through these windows. In vasospasm the vessels are narrowed and flow velocity increases. This is the basis of TCD, which is a noninvasive procedure and can be performed on bedside.

Ultrasonography can be used to detect hydrocephalus and meningomyelocele in prenatal period . USG can also detect hydrocephalus in infant as anterior fontanel is not closed.

Intraoperative USG is used for real time imaging , localization, extent of resection of the tumor after craniotomy at the time of neurosurgery.

Neuronavigation is used to localize the lesion, route of the surgery, safer trajectory, etc.

Neurointervention is a very promising development in the field of Neuro-radiology. It is not only useful for the diagnosis but it also offers to treat many ailments of the brain and spine. The ost important and exciting applications of neurointerventions are: Coiling of the intracranial aneurysms, Preoperative embolization of the vascular tumors like meningioma, Embolization of the intracranial and spinal AVMs, Stenting of the vessel.